Measurement and Parameterization of Spin-Spin Coupling Constants and Potential Applications in Conformational Analysis of Oligosaccharides

Recent studies showed that carbohydrates play critical roles in a lot of biological activities, and conformations of carbohydrates and hydrogen bonds were believed to hold the key to understanding these biological functions of oligosaccharides. Since the pioneer work of Karplus, spin-spin coupling constants (J-coupling constants) had been the most important and reliable tool in conformational analysis.In this thesis, density functional theory (DFT) was used to calculate J-coupling constants involving hydroxyl protons, while systematically changes the dihedral angles of interest. Karplus-like equations paramerized were in good agreement with previously reported equations, in which small non-carbohydrate molecules were used as model compounds. Newly obtained Karplus equations were used to interpret experimentally collected 3JHCOH and 3JCCOH coupling constants in methyl-D-ÌøåÁ- and Ìøå¢-lactoside. Low temperature (- 20 Ìø'_å¡C) and mixed solvent (aceton-d6 and water) were applied to reduce the exchange between hydroxyl protons and bulk solvent. Most H-C-O-H coupling constants measured were in the 4-6 Hz range, which should be expected from a free C-O rotation. However, an exceptionally small coupling (2.8 Hz) for H3'-C3'-O3'-H suggested a rotational bias of C3'-O3' bond. Changing solvent to DMSO reduced the coupling constants further to ~ 1.3 Hz, indicating stronger bias in non-protic solvent. This bias was consistent with the gauche conformation of C3'-O3' in crystal structure and was interpreted as potential hydrogen bonding between O3'-H in the glucose ring and ring oxygen (O5) in the galactose ring. The hydrogen bond was studied theoretically, and its strength (~ 4 kcal/mol) was in line with normal hydrogen bond. A few experiments were designed to find direct evidence of this H-bonding. Some interesting observations were made, however, none of them could firmly prove the persistency of this H-bond.Trans-glycoside linkage coupling pathways C-O-C-H and C-O-C-C are another focus of this thesis. Early parameterizations of C-O-C-H and C-O-C-C assumed similar Karplus behavior as vicinal H-H couplings. However, there is no rigorous theoretical support of this assumption. In this thesis, it is clearly shown that electronegative substituents can significantly affect the Karplus-like behavior of these coupling pathways; hence substituent effects should be considered when interpreting the experimental data. Substituent effects are systematically studied for C-O-C-C coupling constants. The results support the usage of Karplus-like equations for C-O-C-C coupling pathway, with some necessary modifications when electronegative substituents exist.